The area behind your house, especially if it's covered in grass or other plants, is a backyard. Some people have decks or patios in their backyard.
If you live in a city, you'll consider yourself lucky to have a tiny backyard where you can sit on a sunny day. In the suburbs, houses often have identical, fenced-in backyards. And in the country, your backyard might extend for acres. The word is sometimes hyphenated, back-yard, or spelled as two separate words. It's been in use since the 1600s, combining back and yard, from the Old English geard, "enclosure, garden, or court."